In manufacturing thermoplastic resin compositions, it is well known that free, unpolymerized monomer often remains absorbed within the thermoplastic resin produced due to the fact that complete monomer polymerization (particularly monomers such as styrene) is impractical to achieve on an industrial scale. Additionally present may be minor amounts of other organic species such as solvents, oligomers, catalyst, or volatile condensation products. Such substances may be detrimental to the ultimate product formed from the thermoplastic resin by reason of off-taste, off-odor, toxicity, or degradation of physical properties via plasticization, depolymerization, and so forth. Additionally, it is expected that government regulatory agencies may eventually establish maximum permissible levels of various monomers, including styrene, in packaging materials intended to contact food, beverages, pharmaceuticals, and cosmetics on the ground that excess levels represent an unacceptable health risk.
It is also known that the chemical conversion of anhydride groups in a copolymer into cyclic imide groups can provide imide copolymers having higher glass transition temperatures than the corresponding anhydride-containing copolymers, wherein the glass transition temperature increases with increasing imide content. This enhanced transition glass temperature is a very useful and desirable feature for thermoplastic polymers since improved properties such as higher heat distortion resistance and the like can be realized. Various methods have therefore been developed for the reaction of ammonia, amines, or amides with anhydride-containing thermoplastic resins including, for example, the procedures described in U.S. Pat. Nos. 3,840,499 (DiGullo), 4,404,322 (Saito et al.), 4,618,655 (Dehm et al.), and 4,933,395 (Canova et al.).
The development of a process which is capable of simultaneously converting anhydride groups in a thermoplastic resin to imide groups while reducing the level of volatile residues in said resin would therefore be of great interest and utility.